The title of most valuable tech company has been solidly in Apple's hands since 2010, when it passed Microsoft at a valuation around $220 billion. Apple has more than doubled in value since then, and passed Exxon Mobil to become the world's most valuable company in any industry. But recently Apple share have slid, with a fall of more than 20% over the last three months as investors worry that iPhone sales have slowed.

Meanwhile, Alphabet--Google's parent company after a reorganization--has been on a tear. Shares are up 45% in the last year before today, as the company joined Facebook, Amazon, and Netflix as the best performing large cap stocks of 2015. Apple and Alphabet have danced closer in recent weeks, but it was stellar fourth quarter earnings from the search giant that pushed it into the lead on Monday.

Alphabet ended the day at a market capitalization of about $530 billion market cap compared to Apple at just under $535 billion. Shares of Alphabet rose from $770.77 at the close of trading to more than $815 per share after hours, driving the company's market cap above $550 billion.